Method of switching out-of-plane magnetic tunnel junction cells

ABSTRACT

A method of switching the magnetization orientation of a ferromagnetic free layer of an out-of-plane magnetic tunnel junction cell, the method including: passing an AC switching current through the out-of-plane magnetic tunnel junction cell, wherein the AC switching current switches the magnetization orientation of the ferromagnetic free layer.

This is a continuation application of U.S. patent application Ser. No.13/964,402, filed Nov. 16, 2010, now U.S. Pat. No. 8,508,973, thedisclosure of which is incorporated by reference herein in its entirety.

BACKGROUND

New types of memory have demonstrated significant potential to competewith commonly utilized forms of memory. For example, non-volatilespin-transfer torque random access memory (referred to herein as ST-RAM)has been discussed as a “universal” memory. Magnetic tunnel junction(MTJ) cells have has attracted much attention for their application inST-RAM due to their high speed, relatively high density and low powerconsumption.

Most activities have been focused on MTJ cells with in-plane magneticanisotropies. MTJ cells with out-of-plane magnetization orientations arepredicted to be able to achieve lower switching currents than in-planeMTJ cells with the same magnetic anisotropy fields. Therefore,out-of-plane magnetization orientation MTJ cells and methods ofutilizing them are an area of considerable interest.

BRIEF SUMMARY

The present disclosure relates to magnetic spin-torque memory cells,often referred to as magnetic tunnel junction cells, which have magneticanisotropies (i.e., magnetization orientation) of the associatedferromagnetic layers aligned perpendicular to the wafer plane, or“out-of-plane”, and methods of utilizing them.

One particular embodiment of this disclosure is a method of switchingthe magnetization orientation of a ferromagnetic free layer of anout-of-plane magnetic tunnel junction cell, the method including:passing an AC switching current through the out-of-plane magnetic tunneljunction cell, wherein the AC switching current switches themagnetization orientation of the ferromagnetic free layer.

Another particular embodiment of this disclosure is a magnetic memorysystem that includes a magnetic tunnel junction cell having aferromagnetic free layer, a barrier layer, and a ferromagnetic referencelayer, wherein the barrier layer is positioned between the ferromagneticreference layer and the ferromagnetic free layer, and the magnetizationorientation of the ferromagnetic free layer and the ferromagneticreference layer are out-of-plane; and an AC current source electricallyconnected to the magnetic tunnel junction cell.

Yet another particular embodiment of this disclosure is a method ofstoring data electronically, the method including providing anout-of-plane magnetic tunnel junction memory cell, the out-of-planemagnetic tunnel junction memory cell including a ferromagnetic freelayer, a barrier layer, and a ferromagnetic reference layer, wherein thebarrier layer is positioned between the ferromagnetic reference layerand the ferromagnetic free layer, and the magnetization orientation ofthe ferromagnetic free layer and the ferromagnetic reference layer areout-of-plane; and passing an AC switching current through theout-of-plane magnetic tunnel junction cell, wherein the AC switchingcurrent switches the magnetization orientation of the ferromagnetic freelayer, thereby storing a bit of data.

These and various other features and advantages will be apparent from areading of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of thefollowing detailed description of various embodiments of the disclosurein connection with the accompanying drawings, in which:

FIG. 1A is a schematic diagram of an illustrative MTJ cell; FIG. 1B is aschematic diagram of an illustrative MTJ cell that includes an optionalpinning layer; FIG. 1C is a schematic diagram of an illustrative MTJcell with out-of-plane magnetization orientation in a low resistancestate; FIG. 1D is schematic side view diagram of the illustrativemagnetic tunnel junction memory cell in a high resistance state;

FIG. 2A is a schematic diagram illustrating the effect of a directcurrent (DC) switching current on the gyromagnetic relaxation of amagnetization orientation; and FIG. 2B is a schematic diagramillustrating the effect of an alternating current (AC) switching currenton the gyromagnetic relaxation of a magnetization orientation;

FIG. 3 is a schematic diagram of an illustrative memory unit including aMTJ cell and a transistor;

FIG. 4 is a schematic diagram of an illustrative memory array; and

FIG. 5 is a schematic diagram of an illustrative memory system.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

The present disclosure is directed to various embodiments of magnetictunnel junction (MTJ) cells having magnetic anisotropies that result inthe magnetization orientation of the associated ferromagnetic layers tobe aligned perpendicular to the wafer plane, or “out-of-plane”.

In the following description, reference is made to the accompanying setof drawings that form a part hereof and in which are shown by way ofillustration several specific embodiments. It is to be understood thatother embodiments are contemplated and may be made without departingfrom the scope or spirit of the present disclosure. The followingdetailed description, therefore, is not to be taken in a limiting sense.Any definitions provided herein are to facilitate understanding ofcertain terms used frequently herein and are not meant to limit thescope of the present disclosure.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified in all instances by the term“about.” Accordingly, unless indicated to the contrary, the numericalparameters set forth in the foregoing specification and attached claimsare approximations that can vary depending upon the desired propertiessought to be obtained by those skilled in the art utilizing theteachings disclosed herein.

As used in this specification and the appended claims, the singularforms “a”, “an”, and “the” encompass embodiments having pluralreferents, unless the content clearly dictates otherwise. As used inthis specification and the appended claims, the term “or” is generallyemployed in its sense including “and/or” unless the content clearlydictates otherwise.

While the present disclosure is not so limited, an appreciation ofvarious aspects of the disclosure will be gained through a discussion ofthe examples provided below.

FIG. 1A illustrates an exemplary MTJ cell 100 having out-of-planemagnetic orientation. MTJ cell 100 includes a relatively softferromagnetic free layer 110, a ferromagnetic reference (e.g., fixed)layer 140, and an oxide barrier layer 130. Ferromagnetic free layer 110and ferromagnetic reference layer 140 are separated by the oxide barrierlayer 130 or non-magnetic tunnel barrier. The MTJ cell 100 can also bedescribed as having the oxide barrier layer positioned between theferromagnetic reference layer and ferromagnetic free layer.

Free layer 110, and reference layer 140 each have an associatedmagnetization orientation. The magnetization orientations of layers 110and 140 are oriented non-parallel to the layer extension and to theplane of the wafer substrate on which MTJ cell 100 is formed. In someembodiments, the magnetization orientations of layers 110 and 140 can bereferred to as “out-of-plane”. In embodiments, the magnetizationorientations of layers 110 and 140 can be “at least substantiallyperpendicular”. In embodiments, the magnetization orientations of layers110 and 140 can be “perpendicular”. The magnetization orientation offree layer 110 is more readily switchable than the magnetizationorientation of reference layer 140. Other optional layers, such as seedlayers, capping layers, or other layers can be included in the MTJ cell100 even though they are not depicted in these figures.

Free layer 110 and reference layer 140 may independently be made of anyuseful ferromagnetic (FM) material such as, for example, Fe, Co or Niand alloys thereof, such as NiFe and CoFe. Either or both of free layer110 and reference layer 140 may be either a single layer or multilayers.Specific examples of materials that can make up the free layer and thefixed layer can include single layers with perpendicular anisotropy suchas TbCoFe, GdCoFe, and FePt; laminated layers such as Co/Pt Co/Nimultilayers; and perpendicular anisotropy materials laminated with highspin polarization ferromagnetic materials such as Co/Fe and CoFeBalloys. In embodiments, the free layer 110 can include a high spinpolarization layer such as Co and a rare earth-transition metal alloylayer such as GdFeCo. In embodiments, the reference layer 140 mayinclude a high spin polarization layer such as Co and a rareearth-transition metal alloy layer such as TbFeCo.

Barrier layer 130 may be made of an electrically insulating materialsuch as, for example an oxide material (e.g., Al₂O₃, TiO_(x) or MgO_(x))or a semiconductor material. Barrier layer 130 can be a single layer orcan be a layer laminated with another oxide or metal (for example aMg/MgO bilayer). Barrier layer 130 could optionally be patterned withfree layer 110 or with reference layer 140, depending on processfeasibility and device reliability.

FIG. 1B illustrates another exemplary embodiment of a MTJ cell. This MTJcell 101 includes an optional pinning layer 150 disposed proximate, oradjacent to the reference layer 140. The pinning layer 150, if presentpins the magnetization orientation of reference layer 140. In someembodiments, such a pinning layer 150 may have a zero magnetization, butcan still pin the magnetization orientation of the reference layer 140.A pinning layer, if present, may be an antiferromagnetically orderedmaterial (AFM) such as PtMn, IrMn, and others.

FIG. 1C shows a magnetic tunnel junction memory cell 105 in a lowresistance state where the magnetization orientation of free layer 110is in the same direction as the magnetization orientation of referencelayer 140. In FIG. 1D, the magnetic tunnel junction cell 106 is in thehigh resistance state where the magnetization orientation of the freelayer 110 is in the opposite direction of the magnetization orientationof the reference layer 140. In some embodiments, the low resistancestate may be the “0” data state and the high resistance state the “1”data state, whereas in other embodiments, the low resistance state maybe “1” and the high resistance state “0”.

Switching the resistance state and hence the data state of a magnetictunnel junction cell via spin-transfer occurs when a switching current,passing through a magnetic layer of magnetic tunnel junction cellbecomes spin polarized and imparts a spin torque on the free layer 110.When a sufficient spin torque is applied to the free layer 110, themagnetization orientation of the free layer 110 can be switched betweentwo opposite directions and accordingly the magnetic tunnel junctioncell can be switched between the low resistance state and the highresistance state.

Disclosed herein are methods of switching the magnetization orientationof a ferromagnetic free layer of an out-of-plane magnetic tunneljunction cell that includes the step of passing an alternating currentswitching current through the MTJ cell. Alternating current, which canalso be referred to herein as “AC” is an electrical current in which themovement of electric charge (or electrons) periodically reversesdirection. Application of an AC switching current through the MTJ cellswitches (via spin torque as discussed above) the magnetizationorientation of the free layer. In disclosed embodiments, gyromagneticrelaxation also assists in switching the magnetization orientation ofthe free layer. This can afford the use of a lower switching current,thereby allowing the consumption of less power, to be utilized to writedata to an MTJ cell.

Gyromagnetic relaxation of a magnetic field can be described by equation1:{tilde over (τ)}_(g)=({tilde over (M)}×{tilde over (H)})  (Equation 1)where {tilde over (M)} is the magnetization saturation of the free layerand {tilde over (H)} is the magnetic field generated by a current.Conversely, damping relaxation can be described by equation 2:{tilde over (τ)}_(d) =α{tilde over (M)}×({tilde over (M)}×{tilde over(H)})  (Equation 2)where {tilde over (M)} and {tilde over (H)} are as given above and α isabout 0.01. Because of the magnitude of α, the gyromagnetic relaxationis at least about 100 times higher than the damping relaxation in anygiven system. Therefore, if gyromagnetic relaxation can be utilized toassist in switching the magnetization orientation of the ferromagneticfree layer, the overall switching current can be reduced.

The effect of a DC current on gyromagnetic relaxation is schematicallydepicted in FIG. 2A. As seen in FIG. 2A, a DC switching current pullsthe magnetization orientation of the free layer (depicted by the solidarrow) equally away from the center in all directions, therebyeffectively cancelling out the effect of the gyromagnetic relaxation.Conversely, the effect of an AC switching current is depicted in FIG.2B. As seen in FIG. 2B, the gyromagnetic relaxation in this case doesnot cancel itself out and therefore assists in switching themagnetization orientation of the free layer. In embodiments thatutilized an AC switching current, the AC switching current can induce amagnetic field that circumnavigates the magnetic tunnel junction cell,as seen in FIG. 2B. The induced magnetic field can induce gyromagneticrelaxation in the magnetization orientation of the ferromagnetic freelayer. Such gyromagnetic relaxation can contribute to the switching ofthe ferromagnetic free layer. Because of the contribution of thegyromagnetic relaxation to the switching, a lower switching current canbe used, which can afford memory that can function with lower powerrequirements.

In embodiments, the frequency of the AC switching current can be matchedto the gyromagnetic frequency of the ferromagnetic free layer. Thegyromagnetic frequency of the free layer is a function of magneticproperties of the free layer and the geometry of the free layer. Thegyromagnetic frequency is generally in the GHz frequency range.

The AC switching current can be passed from the free layer through thebarrier layer to the reference layer; or from the reference layerthrough the barrier layer to the free layer.

Disclosed methods can optionally further include reading or sensing theresistance state or data of the MTJ cell. The resistance state of a MTJcell can be determined by passing a reading current through the MTJcell. In embodiments, the reading current can be a DC reading current.The measured or sensed resistance (or voltage) can be compared to areference resistance (or voltage). In embodiments, the reading currentcan have an amplitude that is less than the amplitude of the switchingcurrent. In embodiments, a DC reading current that has an amplitude thatis less than the amplitude of the AC switching current can be utilizedfor reading or sensing the resistance of the MTJ cell.

Also disclosed herein are methods of storing data electronically thatinclude providing a disclosed MTJ cell. Providing can includemanufacturing, purchasing, configuring a MTJ cell within a system forstoring data electronically, or other actions. The method can alsoinclude passing an AC switching current through the MTJ cell asdescribed above to switch the magnetization orientation of theferromagnetic free layer. Switching the magnetization orientation of theferromagnetic free layer can function to store a bit of data, either a 0(if the free layer is parallel to the reference layer for example) or a1 (if the free layer is opposite to the reference layer for example).The direction that the AC switching current is passed through the MTJwill dictate whether a 0 or a 1 is stored in the MTJ.

Such a method can also include passing a reading current through the MTJcell to measure or sense the resistance of the MTJ cell. A second (andsubsequent) AC switching current (either in the same or a differentdirection) can also be passed through the MTJ cell either before, after,or both a reading current can be passed through the MTJ cell.

FIG. 3 is a schematic diagram of an illustrative memory unit 300including a MTJ cell 310 electrically connected to a transistor 320,such as a semiconductor based transistor, via an electrically conductingelement 340. MTJ cell 310 may be any of the MTJ cells described herein.Transistor 320 can include a semiconductor substrate 350 having dopedregions (e.g., illustrated as n-doped regions) and a channel region(e.g., illustrated as a p-doped channel region) between the dopedregions. Transistor 320 can include a gate 360 that is electricallycoupled to a word line WL to allow selection and current to flow from abit line BL to MTJ cell 310.

An array of programmable metallization memory units can also be formedon a semiconductor substrate utilizing semiconductor fabricationtechniques. FIG. 4 is a schematic circuit diagram of an illustrativememory array 400. A plurality of memory units 450, described herein canbe arranged in an array to form the memory array 400. The memory array400 can include a number of parallel conductive bit lines 410. Thememory array 400 can also include a number of parallel conductive wordlines 420 that are generally orthogonal to the bit lines 410. The wordlines 420 and bit lines 410 can form a cross-point array where a memoryunit 450 can be disposed at each cross-point. The memory unit 450 andmemory array 400 can be formed using conventional semiconductorfabrication techniques.

Also disclosed herein are memory systems. Disclosed memory systems caninclude a MTJ cell and an AC current source. An exemplary system isschematically depicted in FIG. 5. The magnetic memory system 500 caninclude a MTJ cell 505 that includes a free layer 510, a barrier layer530 and a reference layer 540, as discussed above. The system 500 canalso include an AC current source 501 that is electrically connected tothe MTJ cell 505. Although not depicted herein, a transistor can alsooptionally be electrically connected to the MTJ cell 505. Such systemscan also optionally include a plurality of MTJ cells configured in anarray for example. In such an embodiment, each of the plurality of MTJcells can be electrically connected to the AC current source. Suchsystems can also optionally include a DC current source 502 that iselectrically connected to the MTJ cell 505 (or each of the plurality ofMTJ cells). The DC current source can be utilized to read or sense theresistance of the MTJ cell (or the plurality of MTJ cells).

MTJ cells as disclosed herein can be manufactured using varioustechniques, including for example plasma vapor deposition (PVD),evaporation, and molecular beam epitaxy (MBE).

Methods of switching MTJ cells, methods of storing data, and memorysystems as disclosed herein can be used in MRAM applications.

Thus, embodiments of METHODS OF SWITCHING OUT-OF-PLANE MAGNETIC TUNNELJUNCTION CELLS are disclosed. The implementations described above andother implementations are within the scope of the following claims. Oneskilled in the art will appreciate that the present disclosure can bepracticed with embodiments other than those disclosed. The disclosedembodiments are presented for purposes of illustration and notlimitation, and the present disclosure is limited only by the claimsthat follow.

What is claimed is:
 1. A method of switching the magnetizationorientation of an out-of-plane ferromagnetic free layer, the methodcomprising: passing an AC switching current through the out-of-planeferromagnetic free layer, wherein the AC switching current switches themagnetization orientation of the ferromagnetic free layer.
 2. The methodaccording to claim 1, wherein the frequency of the AC switching currentis matched to the gyromagnetic frequency of the ferromagnetic freelayer.
 3. The method according to claim 2, wherein the AC switchingcurrent induces a magnetic field that circumnavigates the magnetictunnel junction cell.
 4. The method according to claim 3, wherein themagnetic field induces gyromagnetic relaxation in the magnetizationorientation of the ferromagnetic free layer.
 5. The method according toclaim 1, wherein the out-of-plane ferromagnetic free layer is part of amagnetic tunnel junction cell, the magnetic tunnel junction cell furthercomprising a barrier layer and an out-of-plane ferromagnetic referencelayer with the barrier layer positioned between the out-of-planeferromagnetic free layer and the ferromagnetic reference layer, andwherein the AC switching current is passed from the ferromagneticreference layer to the out-of-plane ferromagnetic free layer.
 6. Themethod according to claim 1, wherein the out-of-plane ferromagnetic freelayer is part of a magnetic tunnel junction cell, the magnetic tunneljunction cell further comprising a barrier layer and an out-of-planeferromagnetic reference layer with the barrier layer positioned betweenthe out-of-plane ferromagnetic free layer and the ferromagneticreference layer, and wherein the AC switching current is passed from theferromagnetic free layer to the ferromagnetic reference layer.
 7. Themethod according to claim 1 further comprising passing a DC readingcurrent through the magnetic tunnel junction cell and sensing theresistance of the magnetic tunnel junction cell.
 8. The method accordingto claim 7, wherein the DC reading current has an amplitude that is lessthan that of the AC switching current.
 9. A magnetic memory systemcomprising: at least one magnetic tunnel junction cell having aferromagnetic free layer, a barrier layer, and a ferromagnetic referencelayer, wherein the barrier layer is positioned between the ferromagneticreference layer and the ferromagnetic free layer, and the magnetizationorientation of the ferromagnetic free layer and the ferromagneticreference layer are out-of-plane; an AC current source electricallyconnected to the at least one magnetic tunnel junction cell; and a DCcurrent source electrically connected to the at least one magnetictunnel junction cell.
 10. The magnetic memory system according to claim9, wherein the magnetization orientation of the ferromagnetic free layerand the ferromagnetic reference layer are at least substantiallyperpendicular.
 11. The magnetic memory system according to claim 9,wherein the magnetization orientation of the ferromagnetic free layerand the ferromagnetic reference layer are perpendicular.
 12. Themagnetic memory system according to claim 9 further comprising aplurality of magnetic tunnel junction cells configured in an array. 13.The magnetic memory system according to claim 9 wherein each of theplurality of magnetic tunnel junction cells are electrically connectedto the DC and AC current sources.
 14. The magnetic memory systemaccording to claim 9, wherein the AC current source is configured toaffect the orientation of the ferromagnetic free layer.
 15. The magneticmemory system according to claim 9, wherein the DC current source isconfigured to sense the orientation of the ferromagnetic free layer. 16.A method of storing data electronically comprising: providing anout-of-plane magnetic tunnel junction memory cell, the out-of-planemagnetic tunnel junction memory cell comprising a ferromagnetic freelayer, a barrier layer, and a ferromagnetic reference layer, wherein thebarrier layer is positioned between the ferromagnetic reference layerand the ferromagnetic free layer, and the magnetization orientation ofthe ferromagnetic free layer and the ferromagnetic reference layer areat least substantially perpendicular; and passing an AC switchingcurrent through the out-of-plane magnetic tunnel junction cell, whereinthe AC switching current switches the magnetization orientation of theferromagnetic free layer, thereby storing a bit of data.
 17. The methodaccording to claim 16, wherein the AC switching current is matched tothe gyromagnetic frequency of the ferromagnetic free layer, and the ACswitching current induces a magnetic field that circumnavigates themagnetic tunnel junction cell.
 18. The method according to claim 16,wherein the AC switching current is passed from the ferromagneticreference layer to the ferromagnetic free layer or from theferromagnetic free layer to the ferromagnetic reference layer.
 19. Themethod according to claim 16 further comprising passing a DC readingcurrent through the magnetic tunnel junction cell and sensing theresistance of the magnetic tunnel junction cell.
 20. The methodaccording to claim 16 further comprising: passing a DC reading currentthrough the magnetic tunnel junction cell and sensing the resistance ofthe magnetic tunnel junction cell; and passing a second AC switchingcurrent through the magnetic tunnel junction cell to switch themagnetization orientation of the ferromagnetic free layer a second time.